1. Field of the Invention
The present invention relates to an instrument using scanning charged-particle technology, such as a scanning electron microscope and, more particularly, to a method and apparatus for observing a specimen in a tilted state on a scanning charged-particle beam instrument capable of mechanically moving a specimen and of electrically moving a scanned image.
2. Description of the Related Art
In a scanning electron microscope, an electron beam emitted from an electron gun is focused onto a specimen by condenser lenses and an objective lens. Furthermore, the electron beam is scanned in two dimensions. This results in secondary electrons and other electrons, which are detected by a detector. The output signal from the detector is supplied to a display unit synchronized with the scanning of the electron beam. In this way, a scanned image of the specimen is obtained.
When an image of the specimen is observed using such a scanning electron microscope, the specimen stage is mechanically translated in the X-and Y-directions or rotated to observe an image of a desired region on the specimen. The movement and rotation of the range in which the specimen is observed are not limited to mechanical methods. For example, image shift capabilities for controlling the range in which the electron beam is deflected or scan rotation capabilities for electrically rotating the direction in which the electron beam is scanned in two dimensions may also be employed.
Prior to photographing or recording of a scanned image of a certain portion on a specimen or prior to analysis of the certain portion, an operator always preliminarily observes various portions on the specimen to search for portions adapted for photographing, analysis, or other purpose. FIGS. 1(a) to 1(f) show images observed on a scanning electron microscope (SEM). The operator has just finished a preliminary observation of a scanned image as shown in FIG. 1(a). Then, the operator lowers the magnification of the scanned image as shown in FIG. 1(b), and goes to an operation for searching for a portion to be observed next. The area surrounded by the dotted frame in FIG. 1(b) shows a field of view of FIG. 1(a) that has been observed. This portion is referred to as the portion A herein. It is now assumed that the operator selects a right lower portion of the frame of image as a portion to be observed next within the image of FIG. 1(b). This portion is referred to as the portion B. The operator drives the specimen stage to move the specimen in the direction indicated by the arrow in FIG. 1(b). That is, the specimen is moved toward the upper left corner to bring the portion B into the middle of the viewing screen. The result is shown in FIG. 1(c). Then, the magnification is increased. The result is shown in FIG. 1(d). The operator subsequently considers the compositional arrangement of the image within the frame of image and rotates the image through 90xc2x0 in a clockwise direction. The result is shown in FIG. 1(e). In this way, the operator observes the portions A and B, and then he or she preliminarily observes portions C, D, etc. Finally, he or she selects some portions suitable for the purpose.
However, the operator might want to observe the portion A again during or after observation of the portion B in FIG. 1(e) for some other reason. This can take place frequently.
The operator remembers that he or she moved the specimen stage toward the upper left corner in going from portion A to portion B. Accordingly, he or she may perform a reverse operation, i.e., the specimen stage is moved toward the bottom left corner. However, one would easily understand that the portion A is not resumed by lowering the magnification of the scanned image and the state of FIG. 1(f) is assumed. As shown in FIG. 1(f), the portion A has already rotated through 90xc2x0 in a clockwise direction and so has moved toward the upper right corner. Accordingly, in order to return to the portion A, the specimen stage must be rotated through 90xc2x0 in a counterclockwise direction and then moved toward the lower right corner. Alternatively, the specimen stage must be moved toward the left bottom corner and then rotated through 90xc2x0 in a counterclockwise direction. Such a misunderstanding tends to occur where a rotational operation is involved.
In the description provided thus far, the shown images are such that the portion A can be easily found simply by lowering the magnification of the scanned image as shown in FIG. 1(f) for ease of illustration. On actual specimens, however, scanned images are not viewed as simply as shown in FIGS. 1(a)-1(f). Preliminary observations of portions C, D, etc. are performed, and then the operator wants to return to observations of some earlier portions. Consequently, reproduction is not easy to achieve. In the. description above, the operator observed the portion A in FIG. 1(a) and judged that the portion A was not suitable for the purpose. In spite of this, he or she wants to observe the portion A again. Hence, the reproduction is more difficult to accomplish.
During actual observation of scanned images, the specimen position is shifted by moving the specimen stage in the X-and Y-directions mechanically. If necessary, rotation is added. For fine adjustment, the scanned image is moved in the X-and Y-directions by image shift capabilities. Often, the image is rotated by scan rotation capabilities by taking account of the compositional arrangement of the scanned image.
As mentioned previously, where the operator wants to observe an original image after moving the image, complex operations are often involved.
It is an object of the present invention to provide a method and apparatus for observing specimen images on a scanning charged-particle beam instrument in such a way that a specimen or a specimen stage can be returned to its original observational position automatically and accurately after the specimen or stage is moved.
A method of observing a specimen image on a scanning charged-particle beam instrument in accordance with a first embodiment of the present invention starts with preparing the scanning charged-particle beam instrument having a specimen stage capable of mechanical X-and Y-translations and rotation. A specimen is placed on the specimen stage. A charged-particle beam is directed to the specimen and scanned across the specimen in two dimensions, producing a signal. Based on this signal, a scanned image of the specimen is displayed. A scanning signal for the charged-particle beam is controlled to rotate the direction of the two-dimensional scan (referred to as the scan rotation capabilities). The range in which the charged-particle beam is scanned is shifted (referred to as the image shift capabilities). Then, the amounts of mechanical movements of the specimen in the X-and Y-directions and rotational movement of the specimen in going from a certain observational position to a next observational position are stored in memory. If the image is rotated or shifted at the next observational position using the scan rotation capabilities or image shift capabilities, information about the angular position or amount of shift of the image is stored in memory. If an observation is made at a given position, the image previously obtained at the given position is automatically reproduced, based on stored information about the angular position of the image owing to the scan rotation capabilities, image shift capabilities, and based on information about the amount of shift, the amount of movements in the X-and Y-directions, and the amount of rotation.
A method of observing a specimen image on a charged-particle beam instrument in accordance with a second embodiment of the present invention is based on the first embodiment described above and further characterized as follows. Whenever the specimen is moved from a certain observational position to plural other observational positions, if the image is rotated and shifted at plural observational positions using the amounts of mechanical movements of the specimen in the X-and Y-directions, the amount of rotation, the scan rotation capabilities, and image shift capabilities, then information about the angular position and the amount of shift of the image is stored in memory. Where an observation is made at any one of the plural observational positions, the image at this position is automatically reproduced, based on stored information about the angular position of the image owing to the scan rotation capabilities, image shift capabilities, information about the amount of shift, the amount of movements in the X-and Y-directions, and the amount of rotation. Any one of plural images previously observed can be easily reproduced.
A method of observing a specimen image on a charged-particle beam instrument in accordance with a third embodiment of the present invention is based on the second embodiment described above and further characterized as follows. The history of observational positions is displayed on the image currently being observed. Based on the display of the history, the image at the desired observational position can be easily reproduced.
A fourth embodiment of the present invention provides a scanning charged-particle beam instrument comprising: means for directing a charged-particle beam to a specimen placed on a specimen stage capable of being mechanically moved in the X-and Y-directions and of being rotated; means for scanning the charged-particle beam across the specimen in two dimensions; means for detecting resulting electrons to produce a detector output signal; means for displaying a scanned image of the specimen based on the detector output signal; means having scan rotation capabilities for controlling a scanning signal for the charged-particle beam to rotate the direction of the two-dimensional scan; means having image shift capabilities for shifting a range in which the charged-particle beam is scanned; and a memory for storing the amount of movements in the X-and Y-directions and the amount of rotation of the specimen in going from a certain observational position to a next observational position. If the image is rotated or shifted using the scan rotation capabilities or image shift capabilities at the next observational position, information about the amount of the angular position or the amount of shift is stored in the memory. Where an observation is made at the certain position, the image in the certain position is automatically reproduced, based on stored information about the angular position of the image owing to the scan rotation capabilities, image shift capabilities, information about the amount of shift, the amount of movements in the X-and Y-directions, and the amount of rotation.
A scanning charged-particle beam instrument in accordance with a fifth embodiment of the present invention is based on the fourth embodiment described above and further characterized as follows. Whenever the specimen is mechanically moved from a certain observational position to plural other observational positions, if the image is rotated and shifted at the plural observational positions using the amounts of mechanical movements of the specimen in the X-and Y-directions, the amount of rotation, the scan rotation capabilities, and image shift capabilities, then information about the angular position and the amount of shift of the image is stored in memory. Where an observation is made at any one of the plural observational positions, the image at this position is automatically reproduced, based on stored information about the angular position of the image owing to the scan rotation capabilities, image shift capabilities, information about the amount of shift, the amount of movements in the X-and Y-directions, and the amount of rotation. Any one of plural images previously observed can be easily reproduced.
A scanning charged-particle beam instrument in accordance with a sixth embodiment of the present invention is based on the fifth embodiment described above and further characterized as follows. The history of observational positions is displayed on the image currently being observed. Based on the display of the history, the image at the desired observational position can be easily reproduced.
Other objects and features of the invention will appear in the course of the description thereof, which follows.